1. Field of the Invention
The present general inventive concept relates to an electrophotographic toner, and more particularly to a toner to develop electrostatic latent images.
2. Description of the Related Art
A high level of performance is demanded for printing devices such as printers and copiers. A printer that performs at a high level provides full-color and has a printing speed that is very fast. Such printers are capable of producing high quality images and are compact, inexpensive, and environmentally friendly. In order to meet such a trend, technology for controlling a shape and surface of toner particles while obtaining physical properties of electrophotographic toner has become more important than ever before.
As the printing speed of printers increases, the number of times a shearing force is exerted on toner increases. Thus, demand for high durability of toner is on the rise. To implement a compact and environmentally friendly printer, the amount of “untransferred toner” has to be reduced. In this regard, improvement of charge uniformity and transferability of toner is demanded. In order to obtain high quality print images, improvements in charge stability, transferability, and cleaning ability of toner are required.
Improvements in surface characteristics of toner particles are required to achieve excellent charge uniformity, charge stability, transferability, and cleaning ability with respect to the toner particles. One of the most important factors affecting the surface characteristics of toner particles may be an external additive that is added to the surface of toner particles. One of the main functions of the external additive is to help toner powder to maintain fluidity by preventing the toner particles from attaching to each other. Obviously, an external additive may also affect charge uniformity, charge stability, transferability, and cleaning ability. A silica powder or a titanium oxide powder is mainly used as an external additive.
However, a conventional external additive is known to be unfavorable in terms of obtaining charge uniformity. For example, fumed silica, which is the most commonly used external additive, has a very strong negative polarity. Accordingly, an excessive charge-up phenomenon may frequently occur in a toner that has fumed silica externally added thereto.
A method of additionally externally adding titanium oxide particles to prevent excessive frictional charging due to the excessive charge-up phenomenon caused by the fumed silica has been tried. However, since titanium oxide has low electric resistance and good charge exchangeability, relatively reverse or weak charging toner may be easily produced. Thus, when toner is externally added with silica, charge uniformity of the toner may be reduced.
Silica particles may be porous. Also, silica particles may have hydrophilic surfaces. If silica particles are highly porous and the surfaces thereof are highly hydrophilic, toner that is externally added with the silica particles in a high-temperature and high-humidity environment may not be well charged due to excessive absorption of moisture, which serves as an electrical conductor. On the other hand, toner that is externally added with the silica particles is generally excessively charged in a low-temperature and low-humidity environment. That is, charge stability of toner externally added with the silica particles, which varies depending on the environment, may be deteriorated.
Silica particles or titan oxide particles that are surface-treated with a surface treating agent such as hydrophobic silicone oil or a hydrophobic silica coupling agent may be used as an external additive in order to solve environmental charge stability degradation due to moisture. However, if such external additive particles treated with the surface treating agent are used, cohesiveness between toner particles increases, and thus fluidity of the toner powder may be rapidly degraded.
In a method of manufacturing fumed silica particles, aggregation of the silica particles occurs frequently. The aggregation degrades dispersibility of the fumed silica powder. If an external additive powder with unfavorable dispersibility is used, fluidity, anti-caking ability, fusibility, and cleaning ability of toner obtained as a result may also be degraded.
Sol-gel silica may be used in order to avoid such aggregation of fumed silica. A sol-gel silica powder refers to a silica powder manufactured by using a sol-gel method. For example, the sol-gel silica powder may be obtained by removing a solvent from a sol-gel suspension that is produced by hydrolyzing and condensing alkoxy silane in an organic solvent in which water is present. The sol-gel silica powder manufactured by using the sol-gel method is formed of spherical silica particles with uniform particle size. Conventional sol-gel silica particles have almost a perfect sphere shape. However, if silica particles with a sphericity near 1 are used as an external additive, cleaning ability of toner may be degraded.
Thus, simultaneously improving charge uniformity, charge stability, transferability, and cleaning ability of toner is very difficult.